1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor having a staggered structure and, more particularly, to a method for fabricating a thin film transistor formed on an insulator suitable for application to liquid crystal displays, contact type image sensors, vacuum fluorescent character display and the like.
2. Description of the Related Art
For reduction in costs and for obtaining a high definition function for liquid crystal displays, there is a need for active elements having a higher-speed switching function as compared with conventional hydrogenated amorphous silicon thin film transistors which have been used for driving a liquid crystal display. One such an active element under development is a polysilicon thin film transistor wherein a polysilicon thin film is formed as its active layer by an excimer laser annealing technology. According to the excimer laser annealing technology, a silicon thin film formed on a glass substrate is irradiated by an excimer laser (ultraviolet rays and short-pulsed Laser), thereby melting and recrystallizing only the silicon thin film or annealing-off crystal deflects therein, for obtaining excellent characteristic of a polysilicon thin film. Since the method uses ultraviolet rays and short-pulsed laser, laser energy is absorbed only by the silicon surface without inducing thermal damages on the underlying insulating substrate, thereby annealing the silicon thin film at high temperatures. Accordingly, a general-purpose glass with a low softening point can be used as a substrate material.
Structures of thin film transistors using such a polysilicon thin film include planar type, stagger type and inverted stagger type. Staggered structure transistors (e.g. K. Sera et al., Extended Abstracts of the 1991 International Conference on SSDM (Solid State Devices and Materials), Yokohama, 1991, pp. 590-592) have an advantage of a low leakage current in contrast with transistors of the planar type and other types. FIG. 1 is a cross-sectional view of a conventional thin film transistor of this type using a polysilicon thin film. A gate electrode 5, a gate insulation film 4, a polysilicon active layer 7, an n.sup.+ silicon layer 10 doped with phosphorus and a tungsten silicide (hereinafter referred to as WSi) layer 8 are laminated on a glass substrate 1 to form thin film transistors.
The thin film transistors of this type has advantages of a high-speed function and a low leakage current flowing through source and drain junctions.